This invention relates to a sleeve for use on a print cylinder, and more particularly, it relates to expandable sleeves which are readily mountable onto and dismountable from print cylinders using pressurized air to expand the sleeves for mounting and dismounting. This invention also includes a method and apparatus for forming the sleeves.
In the printing industry, flexible printing "plates" are mounted on sleeves that are readily and repeatably mounted and dismounted from the printing cylinders. The sleeves must be inexpensive, lightweight, resistant to handling damage, and able to withstand several expansions and still reliably grip the print cylinder without slippage (i.e. elastically expandable diametrically). Typically an interference fit with the print cylinder of from 3 to 15 mils is preferred. The sleeves should be expandable with the 40-100 psig air generally available in printing facilities and should expand sufficiently so they are easily slid over the print cylinder, so typically an expansion exceeding the amount of interference fit is required. The sleeves typically have a wall thickness of from about 10 to 40 mils or more.
There is also a need for a sleeve with a precision outer surface which is free of irregularities that cause printing defects and which has a uniform wall thickness that results in a difference in diameter (or trueness, or runout) of the outer wall surface when mounted on a true cylinder of less than 5 mils and preferably less than 1 mil. The surface irregularities must be minimized since the sleeves are occasionally used with printing plates made of a thin photopolymer laminates only 30-125 mils thick mounted with an adhesive layer only 15-20 mils thick. If the printing sleeve is made by helically winding tapes in layers, the tapes must never overlap themselves, and for high quality printing the helical gap between tape edges should be less than 35 mils on the outer surface and preferably less than 20 mils. Gaps at the inner layers are preferably less than 50 mils. A large gap on the outer surface may allow the printing plate to depress into the gap due to print machine pressure as the ink is applied to the plate and transferred from the plate to the paper, thereby producing a poorly printed image on the paper. A large gap on the inner surface may cause excess leakage of the pressurized air for mounting the sleeve.
There is also a need for a sleeve that can be elastically expanded repeatedly to grip the print cylinder firmly to prevent slipping of the sleeve on the cylinder during use. It would be useful to have an expansion of at least 20 mils across the diameter when 40-60 psi fluid pressure is applied (measured by applying pressure to a sealed sleeve acting as a closed cylinder). The elastic expandable sleeve must also have a minimum stiffness across the diameter. There is a need for diametral stiffness for ease of mounting and when the sleeve is dismounted from the print cylinder so the sleeve does not sag with normal handling or during horizontal storage thereby causing distortion in the print plate and sleeve that makes re-mounting the sleeve a problem. The combined features of high expandability and sufficient stiffness are difficult to achieve in a single sleeve structure.
There are many different diameter printing sleeves employed and many different length sleeves. An economical manufacturing process for making sleeves is to make the sleeves on a spiral tube winding machine which permits continuous operation with a simple inexpensive piece of equipment. The machine utilizes a cantilevered, rigid, stationary mandrel on which narrow tapes are guided at an angle to form a helical tubular structure on the mandrel. A drive belt, looped around drive drums and positioned at the same angle to the mandrel, wraps around the tubular tape structure and rotationally and axially propels the structure along the mandrel. At the unsupported end of the mandrel, the tubular structure is periodically cut across its axis to separate the finished product from the still-forming structure.
There is a need for a spiral tube manufacturing process that can provide a quality sleeve for short run lots where the diameter of the sleeve and thickness of the sleeve and hence the helical wrap angle of the sleeve can be readily changed with little waste of raw material or extensive set-up time. Several variables in the helical tube making process contribute to the difficulty of setting up such a process and achieving good running conditions in a short time and with a minimum of poor quality product waste.
Such variables are:
1) variations in the width of the tape supplied;
2) variations in the cut edge quality of the tapes supplied due to poor slitting that produces camber in the edge or edge damage during shipping and handling;
3) variations in the friction between the tapes and the mandrel;
4) variations in the drive belt tension that cause slipping of the belt on the tubular structure or slipping of the belt on the drive drums;
5) variations in the speed of the drive drums for the belt;
6) variations in the angle of the drive belt during operation due to tension variations and slippage, and bending of the mandrel;
7) variations in the tension of the tapes fed to the mandrel;
8) variations in the angle of the tapes fed to the mandrel due to tape wandering (poor tape guiding);
9) inability of the operator to closely monitor the helical gap;
10) wear in the gear trains driving the belt drive drums that causes irregular speed changes in the belt;
11) inaccuracies in the drum diameters and gear train connecting the drums that results in dynamic friction on one drum (more variable than static friction) due to inability to match surface speeds when slightly different diameter drums are geared together.
As for variables due to variations in the speed of the drive drums and slippage of the belt on the drive drums in conventional spiral winding, a downstream drive drum is driven by a motor, and an upstream drive drum is driven by a mechanical linkage to the downstream drum, or is simply an idler. The disadvantages of these configurations are that in the first case employing a mechanical linkage to drive the upstream drum, the peripheral speed of both drive drums are generally not exactly equal due to normal machining and gearing tolerances and therefore the drive belt slides on at least one of the drive drums. This slippage causes a loss of driving power from the slipping drum, and also, potentially, variation in the drive belt speed which could adversely affect the stability of the tube being produced, principally in achieving a tight tolerance small gap between successive wraps. In the second case the upstream drum operates as simply an idler which removes the potential problems of belt slippage due to differential drum speed since the drum is free to rotate at the belt speed; however, the upstream drum applies no additional driving force to the belt and therefore the downstream drum must provide all the driving force, compensate for all torque variations in the tube making process, and rely on the frictional contact between the downstream driven drum and the belt for control. Belt slippage with this arrangement can also be a problem with high torque loads.